CT is a measure of the strength of the disinfectant for the time that the water and disinfectant are in contact.CT is determined by multiplying the residual disinfectant concentration (C) by contact time (T). In order to calculate log inactivation for compliance with the surface water treatment rule the CT Achieved and CT Required must be calculated for each disinfection segment in your system.

Calculating CT Achieved For A Segment

EPA rules state - “Measurements must be taken on the same day of the week, every week, for one year (52 measurements), during peak hourly flow for that day.Data can be measured manually or with on-line instrumentation.”

To meet EPA compliance you must calculate the CT Achieved for a day.There are 2 methods you can choose:

1.calc the CT Achieved for each hour and report the minimum,

calc the CT Achieved for the hour that the Peak Hourly Flow rate (for the segment) occurred.

V1:Hourly Flow Rate in MGD

V100: Clearwell Water Depth (Ft.) Hourly Minimum

V101: Clearwell Volume in MG

Method 1: Calculate CT Achieved for each hour:

Calculate segment volume in MG.

Based on current conditions (i.e. level in tank, number of filters on, etc…)

V101 = V100 * 28.29 * 7.48 / 1000000

Where: 28.29 is Cross-Sectional Area of a 3 foot radius (πr2)

7.48 is number of gallons in 1 cubic foot

1000000 is used to convert gallons to MG

Determine Baffling Factor (T10/T)

Baffling Condition

T10/T

Baffling Description

Unbaffled (mixed flow)

0.1

None, agitated basin, very low length to width ratio, high inlet and outlet flow velocities. Can be approximately achieved in flash mix tank

Calculating CT Required for Segment

Determine which CT Required you need to calculate.Depending on type of disinfectant used and what you are trying to remove (Giardi or Viruses) different equations or lookup tables are used.In this example we will calculate the 3 log (AKA CT99.9) inactivation of Giardi and the 4 log (CT99.99) inactivation of Viruses.

Three (3) Log Inactivation of Giarda by Free Chlorine

There are three methods for calculating the CT Required but they all derive from the CT Tables provided in the EPA Guidance Manual Disinfection Profiling and Benchmarking.All three methods use the Water Temperature, pH, and CL2 Residual:

V121: Water Temp in Deg C (Deg C = (Deg F – 32) * 5/9)

V131:pH

V141:Cl2 Residual at the end of the segment

Method 1: Use WIMS Lookup Tables

You can download the lookup tables here. Unzip files to your \HachWIMS\Client\LookupTables folder.

Method 2: Use EPA Regression Equation

This method is calculated using regression equations developed by Smith et al. (1995). The equations can be found in Appendix E of USEPA Disinfection Profiling and Benchmarking Guidance Manual (EPA 815-R-99-013), August 1999.

This is a method for approximating the CT Tables for 3 Log Inactivation of Giardia Cysts.The CT99.9 calculations for free chlorine use a complex interpolating polynomial which correctly reproduces virtually all of the CT table values for chlorine achieving a coefficient of correlation of .99999.Calculated values vary less than the instruments used to gather the data used in the calculations.The interpolating polynomial for calculating CT99.9 were developed by Russell J. Topham with the Utah Division of Drinking Water as part of his work with water treatment processes under the Surface Water Treatment Rule.

What chlorine and pH values to be used to calculate the required CT (CT99.9) when using multi-segment method to calculating CT, the beginning or the end of segment? The Rule did not specify this clearly. Sometimes, this could make significant difference, particularly for the segment with significant chlorine decay.

Approved: 11/25/2011 8:24 AM

I used the Peak Hour Flow method, with DDWHEN. Simple to follow, works great!

Approved: 5/2/2007 12:53 PM

every time sample changes no homogenicity and no mention of the working strength and range of the disinfectant